A neutron bomb, officially termed as a type of Enhanced Radiation Weapon (ERW), is a low yield thermonuclear weapon in which a burst of neutrons generated by a nuclear fusion reaction is intentionally allowed to escape the weapon, rather than being absorbed by its other components.[3] The neutron burst is used as the primary destructive action of the warhead.

The concept was originally developed by the US in the late 1950s and early 1960s. It was seen as a "cleaner" bomb for use against massed Soviet armored divisions. As these would be used over allied nations, notably West Germany, the reduced blast damage was seen as an important quality.[4][5] Development for the Lance missile was underway in the early 1960s, but abandoned in favor of a conventional design.

ERWs were first operationally deployed for anti-ballistic missiles (ABM). In this role the burst of neutrons would cause nearby warheads to undergo partial fission, preventing them from exploding properly. For this to work, the ABM would have to explode within a few hundred feet of its target. The first example of such a system was the W66, used on the Sprint missile used in the US's Nike-X system. It is believed the Soviet equivalent, the A-135's 53T6 missile, uses a similar design.[6][7]

The weapon was once again proposed for tactical use by the US in the 1970s and 80s, and began production for the Lance in 1981. This time it experienced a firestorm of protest as the growing anti-nuclear movement gained strength through this period. Opposition was so intense that European leaders refused to accept it on their territory. President Reagan bowed to pressure and the built examples of the W70-3 remained stockpiled in the US until they were retired in 1992. The last W70 was dismantled in 2011.[8]

In a standard thermonuclear design, a small fission bomb is placed close to a larger mass of thermonuclear fuel. The two components are then placed within a thick radiation case, usually made from uranium, lead or steel. The case traps the energy from the fission bomb for a brief period, allowing it to heat and compress the main thermonuclear fuel. The case is normally made of depleted uranium or natural uranium metal, because the thermonuclear reactions give off massive numbers of high-energy neutrons that can cause fission reactions in the casing material. These can add considerable energy to the reaction; in a typical design as much as 50% of the total energy comes from fission events in the casing. For this reason, these weapons are technically known as fission-fusion-fission designs.

In a neutron bomb, the casing material is selected to be either transparent to neutrons, or actively enhance their production. The burst of neutrons created in the thermonuclear reaction is then free to escape the bomb, outpacing the physical explosion. By designing the thermonuclear stage of the weapon carefully, the neutron burst can be maximized while minimizing the blast itself. This makes the lethal radius of the neutron burst greater than that of the explosion itself. Since the neutrons disappear from the environment rapidly, such a burst over an enemy column would kill the crews and leave the area able to be quickly reoccupied.

Compared to a pure fission bomb with an identical explosive yield, a neutron bomb would emit about ten times[9] the amount of neutron radiation. In a fission bomb, at sea level, the total radiation pulse energy which is composed of both gamma rays and neutrons is approximately 5% of the entire energy released; in the neutron bomb it would be closer to 40%. Furthermore, the neutrons emitted by a neutron bomb have a much higher average energy level (close to 14 MeV) than those released during a fission reaction (1–2 MeV).[10]

Technically speaking, all low yield nuclear weapons are radiation weapons, including non-enhanced variants. Up to about 10 kilotons in yield, all nuclear weapons have prompt neutron radiation[2] as their furthest-reaching lethal component, after which point the lethal blast and thermal effects radius begins to out-range the lethal ionizing radiation radius.[11][12][13] Enhanced radiation weapons also fall into this same yield range and simply enhance the intensity and range of the neutron dose for a given yield.

Conception of the neutron bomb is generally credited to Samuel T. Cohen of the Lawrence Livermore National Laboratory, who developed the concept in 1958.[14] Initial development was carried out as part of projects DOVE and STARLING, and an early device was tested underground in early 1962. Designs of a "weaponized" version were carried out in 1963.[15][16]

Development of two production designs for the Army's MGM-52 Lance short-range missile began in July 1964, the W63 at Livermore and the W64 at Los Alamos. Both entered Phase 3 testing in July 1964, and the W64 was cancelled in favor of the W63 in September 1964. The W63 was in turn cancelled in November 1965 in favor of the W70 (Mod 0), a conventional design.[15] By this time, the same concepts were being used to develop warheads for the Sprint missile, an anti-ballistic missile (ABM), with Livermore designing the W65 and Los Alamos the W66. Both entered Phase 3 testing in October 1965, but the W65 was cancelled in favor of the W66 in November 1968. Testing of the W66 was carried out in the late 1960s, and entered production in June 1974,[15] the first neutron bomb to do so. Approximately 120 were built, with about 70 of these being on active duty during 1975 and 1976 as part of the Safeguard Program. When that program was shut down they were placed in storage, and eventually decommissioned in the early 1980s.[15]

Development of ER warheads for Lance continued, but in the early 1970s attention had turned to using modified versions of the W70, the W70 Mod 3.[15] Development was subsequently postponed by President Jimmy Carter in 1978 following protests against his administration's plans to deploy neutron warheads to ground forces in Europe.[17] On November 17, 1978, in a test the USSR detonated its first similar-type bomb.[18] President Ronald Reagan restarted production in 1981.[17] The Soviet Union began a propaganda campaign against the US's neutron bomb in 1981 following Reagan's announcement. In 1983 Reagan then announced the Strategic Defense Initiative, which surpassed neutron bomb production in ambition and vision and with that the neutron bomb quickly faded from the center of the public's attention.[18]

Three types of enhanced radiation weapons (ERW) were deployed by the United States.[19] The W66 warhead, for the anti-ICBM Sprint missile system, was deployed in 1975 and retired the next year, along with the missile system. The W70 Mod 3 warhead was developed for the short-range, tactical MGM-52 Lance missile, and the W79 Mod 0 was developed for nuclear artillery shells. The latter two types were retired by President George H. W. Bush in 1992, following the end of the Cold War.[20][21] The last W70 Mod 3 warhead was dismantled in 1996,[22] and the last W79 Mod 0 was dismantled by 2003, when the dismantling of all W79 variants was completed.[23]

According to the Cox Report, as of 1999 the United States had never deployed a neutron weapon. The nature of this statement is not clear; it reads "The stolen information also includes classified design information for an enhanced radiation weapon (commonly known as the "neutron bomb"), which neither the United States, nor any other nation, has ever deployed."[24] However, the fact that neutron bombs had been produced by the US was well known at this time and part of the public record. Sam Cohen suggests the report is playing with the definitions; the US bombs were never deployed to Europe, they remained stockpiled in the US.[25]

In addition to the two superpowers, France and China are known to have tested neutron or enhanced radiation bombs. France conducted an early test of the technology in 1967[26] and tested an "actual" neutron bomb in 1980.[27] China conducted a successful test of neutron bomb principles in 1984 and a successful test of a neutron bomb in 1988. However, neither of those countries chose to deploy the neutron bomb. Chinese nuclear scientists stated before the 1988 test that China had no need for the neutron bomb, but it was developed to serve as a "technology reserve", in case the need arose in the future.[28]

In August, 1999, the Indian government disclosed that India is capable of producing a neutron bomb.[29]

Although no country is currently known to deploy them in an offensive manner, all thermonuclear dial-a-yield warheads that have about 10 kiloton and lower as one dial option, with a considerable fraction of that yield derived from fusion reactions, can be considered able to be neutron bombs in use, if not in name. The only country definitely known to deploy dedicated (that is, not dial-a-yield) neutron warheads for any length of time is the Soviet Union/Russia,[30] which inherited the USSR's neutron warhead equipped ABM-3 Gazelle missile program. This anti-ballistic missile (ABM) system contains at least 68 neutron warheads with a 10 kiloton yield each and it has been in service since 1995, with inert missile testing approximately every other year since then (2014). The system is designed to destroy incoming endoatmospheric level nuclear warheads aimed at Moscow and other targets and is the lower-tier/last umbrella of the A-135 anti-ballistic missile system (NATO reporting name: ABM-3).[7]

The 1979 Soviet/Warsaw pact invasion plan, "Seven Days to the River Rhine" to seize West Germany. Soviet analysts had correctly assumed that the NATO response would be to use regular tactical nuclear weapons to stop such a massive Warsaw Pact invasion.[39] According to proponents, neutron bombs would blunt an invasion by Soviet tanks and armored vehicles without causing as much damage or civilian deaths as the older nuclear weapons would.[40] Neutron bombs would have been used if the REFORGER conventional response of NATO to the invasion was too slow or ineffective.[41][42]

Neutron bombs are purposely designed with explosive yields lower than other nuclear weapons. Since neutrons are scattered and absorbed by air,[2] neutron radiation effects drop off rapidly with distance in air. As such, there is a sharper distinction, relative to thermal effects, between areas of high lethality and areas with minimal radiation doses.[3] All high yield (more than c. 10 kiloton) nuclear bombs, such as the extreme example of a device that derived 97% of its energy from fusion, the 50 megatonTsar Bomba, are not able to radiate sufficient neutrons beyond their lethal blast range when detonated as a surface burst or low altitude air burst and so are no longer classified as neutron bombs, thus limiting the yield of neutron bombs to a maximum of about 10 kilotons. The intense pulse of high-energy neutrons generated by a neutron bomb is the principal killing mechanism, not the fallout, heat or blast.

The inventor of the neutron bomb, Sam Cohen, criticized the description of the W70 as a neutron bomb since it could be configured to yield 100 kilotons:

the W-70 ... is not even remotely a "neutron bomb." Instead of being the type of weapon that, in the popular mind, "kills people and spares buildings" it is one that both kills and physically destroys on a massive scale. The W-70 is not a discriminate weapon, like the neutron bomb—which, incidentally, should be considered a weapon that "kills enemy personnel while sparing the physical fabric of the attacked populace, and even the populace too."[43]

Although neutron bombs are commonly believed to "leave the infrastructure intact", with current designs that have explosive yields in the low kiloton range,[44] detonation in (or above) a built-up area would still cause a sizable degree of building destruction, through blast and heat effects out to a moderate radius, albeit considerably less destruction, than when compared to a standard nuclear bomb of the exact same total energy release or "yield".[45]

Rather than making extensive preparations for battlefield nuclear combat in Central Europe, "The Soviet military leadership believed that conventional superiority provided the Warsaw Pact with the means to approximate the effects of nuclear weapons and achieve victory in Europe without resort to those weapons."[50]

Neutron bombs, or more precisely, enhanced [neutron] radiation weapons were also to find use as strategic anti-ballistic missile weapons,[45] and in this role they are believed to remain in active service within Russia's Gazelle missile.[30]

Wood frame house in 1953 nuclear test, 5 pounds per square inch (psi) overpressure, full collapse. Although neutron bombs, such as that fitted on the MGM-52 Lance missile would cause similar levels of destruction as depicted here within the zone were ~1970s tank crews would also be incapacitated by neutron radiation. When compared to the range of destruction that would be caused by the comparatively higher yield conventional nuclear weapons that it supplanted (e.g., MGR-1 Honest John), which had been needed to deliver the same range and intensity of neutron dose to neutralize tank crews, the range of civilian destruction and amount of fission productfallout generated by a neutron bomb is far more constrained.[51] Sparing the destruction of West Germany more than would otherwise be the case.

Upon detonation, a near-ground airburst of a 1 kiloton neutron bomb would produce a large blast wave and a powerful pulse of both thermal radiation and ionizing radiation, and non-ionizing radiation in the form of fast (14.1 MeV) neutrons. The thermal pulse would cause third degree burns to unprotected skin out to approximately 500 meters. The blast would create at least 4.6 psi out to a radius of 600 meters, which would severely damage all non-reinforced concrete structures. At the conventional effective combat range against modern main battle tanks and armored personnel carriers (< 690–900 m), the blast from a 1 kt neutron bomb would destroy or damage to the point of non-usability almost all un-reinforced civilian buildings.

Using neutron bombs to stop an enemy armored attack by rapidly incapacitating crews with a dose of 8000+ rads of radiation[52] would require exploding large numbers of them to blanket the enemy forces, destroying all normal civilian buildings within c. 600 meters of the immediate area.[52][53]Neutron activation from the explosions could make many building materials in the city radioactive, such as zinc coated steel/galvanized steel (see area denial use below).

Because liquid-filled objects like the human body are resistant to gross overpressure, the 4–5 psi blast overpressure would cause very few direct casualties at a range of c. 600 m. The powerful winds produced by this overpressure, however, could throw bodies into objects or throw debris at high velocity, including window glass, both with potentially lethal results. Casualties would be highly variable depending on surroundings, including potential building collapses.[54]

The pulse of neutron radiation would cause immediate and permanent incapacitation to unprotected outdoor humans in the open out to 900 meters,[9] with death occurring in one or two days. The median lethal dose (LD50) of 600 rads would extend to between 1350 and 1400 meters for those unprotected and outdoors,[52] where approximately half of those exposed would die of radiation sickness after several weeks.

A human residing within, or simply shielded by, at least one concrete building with walls and ceilings 30 cm (12 in) thick, or alternatively of damp soil 24 inches thick, would receive a neutron radiation exposure reduced by a factor of 10.[55][56] Even near ground zero, basement sheltering or buildings with similar radiation shielding characteristics would drastically reduce the radiation dose.[57]

Furthermore, the neutron absorption spectrum of air is disputed by some authorities, and depends in part on absorption by hydrogen from water vapor. Thus, absorption might vary exponentially with humidity, making neutron bombs far more deadly in desert climates than in humid ones.[52]

The neutron cross section and absorption probability in barns of the two natural boron isotopes found in nature (top curve is for 10 B and bottom curve for 11 B. As neutron energy increases to 14 MeV, the absorption effectiveness, in general, decreases. Thus, for boron-containing armor to be effective, fast neutrons must first be slowed by another element by neutron scattering.

The questionable effectiveness of ER weapons against modern tanks is cited as one of the main reasons that these weapons are no longer fielded or stockpiled. With the increase in average tank armor thickness since the first ER weapons were fielded, tank armor protection approaches the level where tank crews are now almost fully protected from radiation effects. Thus, for an ER weapon to incapacitate a modern tank crew through irradiation, the weapon must now be detonated at such a close proximity to the tank that the nuclear explosion's blast would now be equally effective at incapacitating it and its crew.[58] However this assertion was regarded as dubious in a reply in 1986[59] by a member of the Royal Military College of Science as neutron radiation from a 1 kiloton neutron bomb would incapacitate the crew of a tank with a protection factor of 35 out to a range of 280 meters, but the incapacitating blast range, depending on the exact weight of the tank, is much less, from 70 to 130 meters. However although the author did note that effective neutron absorbers and neutron poisons such as boron carbide can be incorporated into conventional armor and strap-on neutron moderating hydrogenous material (substances containing hydrogen atoms), such as explosive reactive armor, can both increase the protection factor, the author holds that in practice combined with neutron scattering, the actual average total tank area protection factor is rarely higher than 15.5 to 35.[60] According to the Federation of American Scientists, the neutron protection factor of a "tank" can be as low as 2,[2] without qualifying whether the statement implies a light tank, medium tank, or main battle tank.

A composite high density concrete, or alternatively, a laminated Graded Z shield, 24 units thick of which 16 units are iron and 8 units are polyethylene containing boron (BPE), and additional mass behind it to attenuate neutron capture gamma rays, is more effective than just 24 units of pure iron or BPE alone, due to the advantages of both iron and BPE in combination. Iron is effective in slowing down/scattering high-energy neutrons in the 14-MeV energy range and attenuating gamma rays, while the hydrogen in polyethylene is effective in slowing down these now slower fast neutrons in the few MeV range, and boron 10 has a high absorption cross section for thermal neutrons and a low production yield of gamma rays when it absorbs a neutron.[61][62][63][64] The Soviet T72 tank, in response to the neutron bomb threat, is cited as having fitted a boronated[65] polyethylene liner, which has had its neutron shielding properties simulated.[56][66]

The radiation weighting factor for neutrons of various energy has been revised over time and certain agencies have different weighting factors, however despite the variation amongst the agencies, from the graph, for a given energy, A fusion neutron (14.1 MeV) although more energetic, is less biologically harmful as rated in Sieverts, than a fission generated thermal neutron or a fusion neutron slowed to that energy, c. 0.8 MeV.

However, some tank armor material contains depleted uranium (DU), common in the US's M1A1 Abrams tank, which "incorporates steel-encased depleted uranium armour",[67] a substance that will fast fission when it captures a fast, fusion-generated neutron, and thus on fissioning will produce fission neutrons and fission products embedded within the armor, products which emit among other things, penetrating gamma rays. Although the neutrons emitted by the neutron bomb may not penetrate to the tank crew in lethal quantities, the fast fission of DU within the armor could still ensure a lethal environment for the crew and maintenance personnel by fission neutron and gamma ray exposure,[68] largely depending on the exact thickness and elemental composition of the armor—information usually hard to attain. Despite this, Ducrete—which has an elemental composition similar to, but not identical to the ceramic 2nd generation heavy metal Chobham armor of the Abrams tank—is an effective radiation shield, to both fission neutrons and gamma rays due to it being a graded Z material.[69][70] Uranium, being about twice as dense as lead, is thus nearly twice as effective at shielding gamma ray radiation per unit thickness.[71]

As an anti-ballistic missile weapon, the first fielded ER warhead, the W66, was developed for the Sprint missile system as part of the Safeguard Program to protect United States cities and missile silos from incoming Soviet warheads by damaging their electronic components with the intense neutron flux.[45]Ionization greater than 5,000 rads in silicon chips delivered over seconds to minutes will degrade the function of semiconductors for long periods.[72] Due to the rarefied atmosphere encountered high above the earth at the most likely intercept point of an incoming warhead by a neutron bomb/warhead – the terminal phase point (10–30 km) of the incoming warhead's flight—the neutrons generated by a mid- to high-altitude nuclear explosion (HANE) have an even greater range than that encountered after a low altitude air burst. In the high altitude case, there is a lower density of air molecules that produces, an appreciable reduction in the air shielding effect/half-value thickness.

However, although this neutron transparency advantage attained only increases at increased altitudes, neutron effects lose importance in the exoatmospheric environment, being overtaken by the range of another effect of a nuclear detonation, at approximately the same altitude as the end of the incoming missile's boost phase (c. 150 km), ablation producing soft X-rays are the chief nuclear effects threat to the survival of incoming missiles and warheads rather than neutrons.[73] A factor exploited by the other warhead of the Safeguard Program, the enhanced (X-ray) radiation W71 and its USSR/Russian counterpart, the warhead on the A-135 Gorgon missile.

Another method by which neutron radiation can be used to destroy incoming nuclear warheads is by serving as an intense neutron generator and to thus initiate fission in the incoming warhead's fissionable components by fast fission[citation needed], potentially causing the incoming warhead to prematurely detonate in a fizzle if within sufficient proximity, but in most likely interception ranges, requiring only that enough fissionable material in the warhead fissions to interfere with the functioning of the incoming warhead when it is later fuzed to explode (see related physics: Subcritical reactor).

Lithium-6 hydride (Li6H) is cited as being used as a countermeasure to reduce the vulnerability and "harden" nuclear warheads from the effects of externally generated neutrons.[74][75]Radiation hardening of the warhead's electronic components as a countermeasure to high altitude neutron warheads somewhat reduces the range that a neutron warhead could successfully cause an unrecoverable glitch by the transient radiation effects on electronics (TREE) effects.[76][77]

In November 2012, during the planning stages of Operation Hammer of God, British Labour peer Lord Gilbert suggested that multiple enhanced radiation reduced blast (ERRB) warheads could be detonated in the mountain region of the Afghanistan/Pakistan border to prevent infiltration.[78] He proposed to warn the inhabitants to evacuate, then irradiate the area, making it unusable and impassable.[79] Used in this manner, the neutron bomb(s), regardless of burst height, would release neutron activated casing materials used in the bomb, and depending on burst height, create radioactive soil activation products.

In much the same fashion as the area denial effect resulting from fission product (the substances that make up most fallout) contamination in an area following a conventional surface burst nuclear explosion, as considered in the Korean War by Douglas MacArthur, it would thus be a form of radiological warfare – with the difference that neutron bombs produce half, or less, of the quantity of fission products relative to the same-yield pure fission bomb. Radiological warfare with neutron bombs that rely on fission primaries would thus still produce fission fallout, albeit a comparatively cleaner and shorter lasting version of it in the area if air bursts were used, as little to no fission products would be deposited on the direct immediate area, instead becoming diluted global fallout.

However the most effective use of a neutron bomb with respect to area denial would be to encase it in a thick shell of material that could be neutron activated, and use a surface burst. In this manner the neutron bomb would be turned into a salted bomb; a case of zinc-64, produced as a byproduct of depleted zinc oxide enrichment, would for example probably be the most attractive for military use, as when activated, the zinc-65 so formed is a gamma emitter, with a half life of 244 days.[80]

Neutron bombs-warheads require considerable maintenance for their abilities, requiring some tritium for fusion boosting[citation needed] and tritium in the secondary stage (yielding more neutrons), in amounts on the order of a few tens of grams[81] (10–30 grams[82] estimated). Because tritium has a relatively short half-life of 12.32 years (after that time, half the tritium has decayed), it is necessary to replenish it periodically to keep the bomb effective. (For instance: to maintain a constant level of 24 grams of tritium in a warhead, about 1.3 grams per bomb per year[83] must be supplied.) Moreover, tritium decays into helium-3, which absorbs neutrons[84] and will thus further reduce the bomb's neutron yield.

^ ab"On this Day: 7 April". BBC. 1978-04-07. Retrieved 2010-07-02. Jimmy Carter's successor, Ronald Reagan, changed US policy and gave the order for the production of neutron warheads to start in 1981. ...

^Christopher Ruddy (June 15, 1997). "Bomb inventor says U.S. defenses suffer because of politics". Tribune-Review. Retrieved 2010-07-03. With the fall of the Berlin Wall and the end of communism as we knew it, the Bush administration moved to dismantle all of our tactical nuclear weapons, including the Reagan stockpile of neutron bombs. In Cohen's mind, America was brought back to Square One. Without tactical weapons like the neutron bomb, America would be left with two choices if an enemy was winning a conventional war: surrender, or unleash the holocaust of strategic nuclear weapons.

^http://web.ead.anl.gov/uranium/pdf/ducretecosteffec.pdf Paper Summary Submitted to Spectrum 2000, Sept 24-28, 2000, Chattanooga, TN. Ducrete: A Cost Effective Radiation Shielding Material. Quote- "The Ducrete/DUAGG replaces the conventional aggregate in concrete producing concrete with a density of 5.6 to 6.4 g/cm3 (compared to 2.3 g/cm3 for conventional concrete). This shielding material has the unique feature of having both high Z and low Z elements in a single matrix. Consequently, it is very effective for the attenuation of gamma and neutron radiation ..."

^"Nuclear Matters Handbook". Nuclear weapon-generated X-rays are the chief threat to the survival of strategic missiles in-flight above the atmosphere and to satellites ... The Neutron and gamma ray effects dominate at lower altitudes where the air absorbs most of the X-rays.

^"Section 12.0 Useful Tables Nuclear Weapons Frequently Asked Questions". Due to moderating ability and light weight, used to harden weapons against outside neutron fluxes (especially in combination with Li-6) ...The very high cross section of this reaction for thermalized neutrons, combined with the light weight of the Li-6 atom, make it useful in the form of lithium hydride for hardening of nuclear weapons against external neutron fluxes.

^After a year the initial amount of 24 grams of Tritium decays to 2^(-1/12.32)x 24=22.68 grams.

^When absorbing neutrons, helium-3 produces back some tritium, but it comes too late in the reaction for fusion boosting and does not compensate for the decayed tritium missing at the reaction start.[citation needed]